Optical waveguide with unequally spaced lenses

ABSTRACT

A beam waveguide for guiding optical wave energy over extended distances by means of lenses of variable spacing whose focal lengths are a function of the separation from adjacent lenses and the maximum lens-to-lens spacing in the system.

I v V United Stat 1151 3,653,738 Gloge 1 51 Apr. 4, 1972 [541 OPTICAL WAVEGUIDE WITH 3.382.022 5/1968 Fox ..3$0/96 wc x UNEQUALLY SPACE!) LENSES 3,410,623 ll/l968 Kogelnik 350/96 WG [73] Assignee: Bell Telephone Laboratories, Incorporated,

Murray Hill, NJ.

[22] Filed: June 29,1970

[21] Appl.No.: 50,569

[52] us. 01 ..350/96 we, 350/294 511 1111.01 ,1 1602b 27/00, HOlp 3/00 581 Field ofSearch "350/96 we, 294

Primary Examiner-John K. Corbin Attorney-R. J. Guenther and Arthur J. Torsiglieri 57 ABSTRACT A beam waveguide for guiding optical wave energy over extended distances by means oflenses of variable spacing whose focal lengths are a function of the separation from adjacent lenses and the maximum lens-to-lens spacing in the system,

5 Claims, 3 Drawing Figures PATENTEDAPR 4:912 3,653,738

TRANSMITTER lNl/ENTOR y D. C. GLOGE ATTORNE V OPTICAL WAVEGUIDE WITH UNEQUALLY SPACED LENSES BACKGROUND OF THE INVENTION The invention of the laser has served to revive interest in light as a communication carrier. However, before a light communication system can become economically and technically feasible, the problems ofguided propagation of light over extended distances must be solved. To this end, a variety of beam waveguides have been proposed. See, for example, US. Pat. Nos. 3,442,574; 3,403,956; and 3,457,001.

For the sake of system simplicity and lowest cost, it would be highly desirable to construct a beam waveguide using equally spaced focusing members (lenses) of the same kind. However, topographic and other right-of-way obstructions may make this impossible.

It is, accordingly, the broad object of the present invention to guide optical wave energy over extended distances by means of arbitrarily spaced focusing members.

SUMMARY OF THE INVENTION In accordance with the present invention, beam guidance is achieved by means of a plurality of arbitrarily spaced lenses whose focal lengths are a function of the distances between adjacent lenses and the maximum lens-to-lens spacing in the system. In a waveguide so proportioned, a coherent Gaussian beam propagating therealong will have the same beam diameter at each ofthe arbitrarily spaced lenses.

It is, thus, an advantage of the invention that all, or portions, of a long distance beam waveguide can be efficiently designed to accommodate limitations imposed by the terrain through which the waveguide must pass.

These and other objects and advantages, the nature of the present invention, and its various features, will appear more fully upon consideration of the various illustrative embodiments now to be described in detail in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows, in block diagram, a typical beam transmission system;

FIG. 2 shows a beam waveguide of arbitrarily spaced focusing members, in accordance with the invention; and

FIG. 3 shows a portion ofa beam waveguide employing gas and periscopic lenses.

DETAILED DESCRIPTION Referring to the drawings, FIG. 1, included for purposes of explanation, shows an optical transmission system comprising a transmitter 10, a receiver 11 and a guiding wavepath 12 connecting the transmitter to the receiver. In the communication system contemplated by the invention, the distance between the transmitter and receiver is long, typically of the order of many miles and, hence, there will generally be many changes in direction in order to conform to the available right-of-ways, terrain undulations, and to avoid natural obstructions. Thus wavepath I2 is shown to include a plurality of straight sections of waveguide 13, arranged at varying orientations, and a plurality of beam deflectors and refocusers 14 for refocusing and redirecting the optical beam from along one of said straight sections of waveguide to along the next adjacent straight section of waveguide.

The general requirements of an optical beam waveguide have been fully described in the literature. See, for example, Effect of Redirectors, Refocusers, and Mode Filters on Light Transmission Through Aberrated and Misaligned Lenses," by E. A. I. Marcatili, published in the Oct. 1967 issue of the Bell System Technical Journal, pp. 1733-1752.

For the sake of simplicity and cost reduction it would be most advantageous if the focusing members comprising the beam waveguide are all the same. That is, all of the lenses (or focusing mirrors), are the same size and have the same focal 5 length. This would require, however, that the lens-to-lens spacings be equal throughout the system. In terms of the wavepath shown in FIG. 1, this would require either that waveguide sections 13 be equal in length or that they differ by integral multiples of the lens-to-lens spacing. Unfortunately,

0 this is not always possible. In the most general case, the lensto-lens spacings will have to be varied in order to conform the wavepath to the terrain. Such a variation, however, would cause the beam to increase in size and, ultimately, to exceed the guide diameter unless the focal lengths of the individual lenses are adapted to the variations in the lens-to-lens spacings. Such a portion of waveguide is illustrated in FIG. 2, which comprises a plurality of arbitrarily spaced lenses 20, 21, 22, 23, 24, and 25. Designating lens 23 as the n' lens in the 2 waveguide, adjacent lenses 22 and 24 are then the (n---] and the (n+1 lens, respectively. The lensto-lens spacing between lenses 22 and 23 is given as d,, while the lens-tolens spacing between lenses 23 and 24 is given as d,,.,. Also given as the distance between lenses 20 and 21 is the max- 25 imum lens-to-lens spacing d for the portion of waveguide illustrated.

In accordance with the present invention, the focal length F,, of the n lens in the waveguide is given as 35 7 W711 be noted that there are two permissible focal lengths,

0 focal lengths. In either case, however, a coherent Gaussian beam, propagating along such a waveguide will maintain a constant 1/e radius of dA/rr at each lens, where A is the free space wavelength of the beam.

While the focusing members have been referred to and illustrated as dielectric lenses, it will be recognized that, alternatively, other focusing means such as gas lenses, pairs of periscopically aligned mirrors, as disclosed in the copending application Ser. No. 887,262, filed Dec. 22, I969, and assigned to applicant's assignee. or combinations thereof, as illustrated in FIG. 3, can be used. As shown in this figure, a gas lens 30 is combined with a periscopic lens 31. Thus, in all cases, it is understood that the above-described arrangement is illustrative of but one of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can readily be devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.

I claim:

1. A beam waveguide comprising:

a plurality of spaced focusing members characterized in that:

the spacings between at least one pair of said members is different than the spacing between another pair of said members;

and in that the focal length of said members are selected to maintain the same beam diameter at each of said members;

W said focal lengths being given by where d and d,, are the member-to-member spacings ing members are dielectric lenses.

between the n' focusing member and the two adjacent 4. The waveguide according to claim 1 wherein said focusfocusing members, and d is the maximum member-toing members are pairs of periscopically aligned mirrors. member spacing along said waveguide. 5. The waveguide according to claim 1 wherein said focus- 2. The waveguide according to claim 1 wherein said focus- 5 ing members comprise combinations of lenses and periscopiing members are gas lenses. Cally aligned mirrors.

3. The waveguide according to claim 1 wherein said focusa a a 

1. A beam waveguide comprising: a plurality of spaced focusing members characterized in that: the spacings between at least one pair of said members is different than the spacing between another pair of said members; and in that the focal length of said members are selected to maintain the same beam diameter at each of said members; said focal lengths being given by where dn 1 and dn 1 are the member-to-member spacings between the nth focusing member and the two adjacent focusing members, and d is the maximum member-to-member spacing along said waveguide.
 2. The waveguide according to claim 1 wherein said focusing members are gas lenses.
 3. The waveguide according to claim 1 wherein said focusing members are dielectric lenses.
 4. The waveguide according to claim 1 wherein said focusing members are pairs of periscopically aligned mirrors.
 5. The waveguide according to claim 1 wherein said focusing members comprise combinations of lenses and periscopically aligned mirrors. 